go/client: make test more resilient (google#1361)

Only fail the retry timing test if the difference in timings is
both:
 - more than 100ms out
 - more than 10% out.

Hopefully this will reduce test flakes.

Author: daviddrysdale

go/client/logclient_test.go

@@ -226,10 +226,11 @@ func TestAddChainRetries(t *testing.T) {
 	if err != nil {
 		t.Fatalf("Failed to create client: %v", err)
 	}
-	leeway := time.Millisecond * 100
-	instant := time.Millisecond
-	fiveSeconds := time.Second * 5
-	sevenSeconds := time.Second * 7 // = 1 + 2 + 4
+	const leeway = time.Millisecond * 100
+	const leewayRatio = 0.1 // 10%
+	const instant = time.Millisecond
+	const fiveSeconds = time.Second * 5
+	const sevenSeconds = time.Second * 7 // = 1 + 2 + 4
 
 	tests := []struct {
 		deadlineLength        int
@@ -258,7 +259,9 @@ func TestAddChainRetries(t *testing.T) {
 		started := time.Now()
 		sct, err := client.AddChain(deadline, chain)
 		took := time.Since(started)
-		if math.Abs(float64(took-test.expected)) > float64(leeway) {
+		delta := math.Abs(float64(took - test.expected))
+		ratio := delta / float64(test.expected)
+		if delta > float64(leeway) && ratio > leewayRatio {
 			t.Errorf("#%d Submission took an unexpected length of time: %s, expected ~%s", i, took, test.expected)
 		}
 		if test.success && err != nil {